In a capacitance-type switch a movable key acts to increase capacitance, and the change in capacitance is then sensed for switching action. Commercially available capacitance switches typically involve changes in capacitance on the order of one to two. The pulses generated by the change in capacitance are typically of short (i.e., nanosecond) duration and are small in amplitude and must be amplified. Also, because the pulses are small, means must be taken to shield noise from external sources.
Prior to the conception of my invention claimed herein, Norman J. Frame invented a capacitance switch construction (FIG. 1) having a capacitance spacer layer of large area to achieve a large difference between unactuated and actuated capacitances, for a greater signal-to-noise ratio. My attempts to practicably sense activation and deactivation of this switch by connecting it to ground and a D.C. source via a pair of resistors and gates to sense voltage changes were frustrated by the amount of time required to discharge the charged capacitor after releasing the switch.